Which is probably exactly why this was added. The cheap way to usually tell if someone is using a 3rd party UI toolkit, is to start tweaking the system theming

Which is probably exactly why this was added. The cheap way to usually tell if someone is using a 3rd party UI toolkit, is to start tweaking the system theming and see if the application follows some scaling/color changes correctly. In this case some subset of apple provided apps weren't following the theme and they fixed it by adding a private css property. Vs some other OS vendor that likely removed most of the theme controls so they didn't have to keep fixing a huge pile of 1/2 baked abandoned toolkits scattered across their product portfolio.